


Crossing the Line

by nihilistshiro



Series: Close Encounters [1]
Category: Voltron: Legendary Defender
Genre: Alien Sex, Alpha/Beta/Omega Dynamics, Altean Shiro, Anal Fingering, Galra Keith, Kinktober, Knotting, M/M, Masturbation, Mating Cycles/In Heat, NSFW, Omega Keith, Omega Verse, SHEITH - Freeform, Scenting, Tails, Xenophilia, alpha shiro
Language: English
Status: Completed
Published: 2018-10-23
Updated: 2018-10-23
Packaged: 2019-08-06 08:21:31
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,831
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16384595
Author URL: https://archiveofourown.org/users/nihilistshiro/pseuds/nihilistshiro
Summary: In which Shiro is an alpha with the Altean special guard and Keith is an omega who hides as a beta working for the Blade of Marmora. They’ve gone on a lot of missions together and can hardly stand to be around one another, always fighting over how to best complete their assignments.But what happens when they get stranded together and Keith goes into an unexpected heat?I think you know.





	Crossing the Line

**Author's Note:**

> *slaps this down, screaming*
> 
> YASSSSSSSSSSSSSS

Shiro dropped the wood he was carrying. The scent hit him like a punch in the gut, knocking the wind out of him and stopping him in his tracks.

Thick and sweet and sumptuous, watering his mouth, his heart hammering in his chest.

Omega.

A growl rumbled in his chest, his lip curling over pearly white fangs.

He'd always assumed Keith was a beta, mostly because, during their several missions together, he didn't scent at all and he was too small to be an alpha. Never would Shiro have guessed the Blade of Marmora agent was an omega.

His hand was glowing without him activating it, the eerie purple prosthetic curling into a tight fist. Shiro took deep breaths, willing the beast within him to back off, but damn. He couldn't remember the last time an omega affected him so.

Shiro waited until the burning in his lungs was manageable and his anger at his Galra companion was tucked away. As a commander with the Altean special guard, Shiro was trained to deal with any situation. The unit only accepted alphas, and part of their training was dealing with omega’s in heat. It was part of his job to prepare for any emergency situation.

They were stranded on a planet after a botched reconnaissance mission and wouldn’t get rescue transport for another few vargas.

This definitely classified as an emergency.

He opened the door to the pod and found the main cabin empty, Keith’s scent coating his tongue as it poured from the closed door to their sleep chambers.

Shiro wanted to go in there. As much as he hated Keith and his hotheaded nature, his hackles rose and arousal pummeled him. They worked together on a regular basis, part of the coalition uniting Altean and Galran forces after the Great War, and each time it didn’t take long for them to get at each other’s throats. Shiro was too by-the-book, too strict, while Keith was reckless and spontaneous, their operating styles like oil and water.

But Shiro knew the omega must be in terrible pain. There was no question that going in would jeopardize the tight leash he held on his instincts, so Shiro stood in the middle of the common space, frozen and unsure, thoughts spiraling in his own uncertainties.

When he could no longer stand the pungent aroma stinging his nostrils, Shiro wrapped on the door.

“Keith?”

He waited but no answer came.

Frustrated and worried, he tapped at the lock pad, the door springing open and a wave of scent crashed over Shiro. He felt like he was sinking into a hot spring, Keith’s scent enveloping him as heat pooled deep in his belly. Shiro was drowning and it took him a moment to clear the red haze from his vision, his eyes adjusting to the low, blue lights glowing in the chamber from the backup generator.

There was a rustling in the nest of pillows and blankets piled on the lower bunk, and Shiro made his way slowly over to it, ignoring the way his mouth watered as the tang of omega coiled around him.

“Keith?” he asked again, not willing to risk touching the nest, but hands outstretched nonetheless.

A low moan greeted him, and Shiro’s pointed ears flicked as he strained to listen, sounds coming from within the heap of fabric.

“ _Shiro._ ”

Shiro stilled, finally picking up on the sounds of Keith, of fingers slicking along wet skin. It occurred to him too late that his presence would cause the omega to start slicking, and soon the scent of it rose like a blaze around him, burning his control to the ground.

He snatched the top blanket off Keith, a growl erupting from him when he saw the naked omega, writhing in the bedding, torso pressed into the mattress, ass up in the air. Keith’s tail was tucked beneath him, circled around his cock and stroking feverishly, cum sticking to his skin from previous rounds. His fingers were pumping in and out of his exposed ass, his body quaking from exertion.

“Keith,” Shiro said, voice low and strained. “How long have you been like this? What happened?”

“Nnnnggghhh!” Keith ground out, panting as he stopped pleasuring himself long enough to roll over and gaze up at Shiro, eyes hooded and glazed over. “Since you left this morning. Suppressants ran out. First heat in–hnnn–five years.”

His hips writhed into thin air, seeking friction, his entire form covered in a thin layer of dewy sweat. Keith’s long, dark hair was wild, pulling from his braid as whisps stuck to the side of his neck and brow.

“Please, help me, Shiro,” Keith managed to mutter through the heavy fog of his heat.

When he felt the suppressants wear off earlier, he’d been grateful to be alone. Then the pangs of need started wracking his lean frame and he tried to placate his aching body while he waited for Shiro to return. The alpha had been the bane of his existence since the day they met. The suppressants Keith took hid his scent, spared him his heat cycle, but they couldn’t dampen his body’s response to the Altean and that pissed Keith off to no end.

Now, Shiro’s scent was suffocating, the lust tinging the musky fragrance making Keith’s legs fall apart instinctively. He watched as Shiro approached the bed, still dressed in his white bodysuit, the material stretched tight over his massive frame. Although Keith was an omega, he was larger than an Altean omega and more muscular to boot. But Shiro was still taller and broader, his muscles thicker and his shoulders wider. The thought of Shiro’s larger body pinning him to the bed made the omega reach down and palm himself.

“You gonna help or not,” he managed to huff out, ears flattening against his skull, body screaming to be filled by an alpha.

“Can you cut it with the shitty attitude for once? I should have known that even when you need something from me, you can’t be civil.”

Keith hissed through his teeth, back arching off the bed as a new, frenzied wave of desire burst beneath his skin. He glared up at the Altean, who was still completely dressed, even if a high blush littered his cheeks, contrasting with his purple markings.

“You just hate me because I’m Galra. Always have.”

“That’s ridiculous,” Shiro countered, anger seeping into his tone. “The Galra gave me my arm after the accident. I hate you because you’re an entitled little shit who can’t follow orders.”

“Better than being a snotty old man with a stick up his ass.”

Shiro’s hand lit up and he punched the wall, the metal crumpling beneath him. They fell into silence, the sound of the impact ringing out as they stared each other down, a palpable tension fogging the air between them. Their scents mingled together, creating an intoxicating perfume that permeated the entire space.

“Why did you come in here if you aren’t going to fuck me?” Keith finally said rolling back over and serving his ass up like a feast. “You can spank me if you want, alpha. Just get over here and put me out of my misery.”

Takashi crossed the room in two strides, palm landing with a resounding smack against Keith’s round bottom, the lavender skin adopting a rosy hue. Keith moaned and waited for the next slap, bracing himself as Shiro’s hand came down again, stinging on the opposite cheek, the pain melding with pleasure to send Keith’s eyes rolling back into his skull.

Instead of another slap, Takashi dragged the gland at his wrist between Keith’s thighs, finding the glands there. Keith let out a ragged sigh as relief washed over him, their oils mixing as the alpha’s touch soothed some of the fever raging within him.

Shiro leaned down, his hands coming up to spread Keith’s cheeks so he could run his tongue along the omega’s perineum. Keith’s tail whipped back and forth before wrapping around Shiro’s wrist, his hips tilting to give Shiro better access to his entrance.

Syrupy sweet slick wet Shiro’s mouth as he worked Keith open, thrusting his tongue in as far as he could, his free hand reaching between Keith’s parted thighs to grip his erection, stroking it in time with the determined movements of his mouth.

“Fuck, that feels good, Shiro,” Keith praised, face burrowed against a pillow, drool pooling in the fabric as Shiro stretched him, his thumb rubbing the muscle of Keith’s hole and dipping in alongside his tongue.

“Fuuu--- _nnnnggghh!_ ” Keith sobbed as Shiro continued his assault, his pace increasing with the omega’s desperation.

Shiro briefly thought of how this was violating procedure. Of what his commander would say when he returned, reeking of omega. Of how the rescue crew might show up at any minute.

The last thought has his inner-alpha rankling. There was an omega beneath him and that omega was his. To have anyone else interfere and see Keith like this–writhing under Shiro’s mouth and hands–would no doubt cause a fight to break out.

But the chaotic spiral of his anxieties evaporated when Keith sighed his name.

“ _Takashi._ ”

Shiro pulled away abruptly, flipping Keith onto his back and crawling up the omega’s body to position himself between Keith’s thighs.

“You’re still dressed,” Keith said, his hoarse voice laced with amusement. “Is this your first time?”

“Quiznak,” Shiro muttered while Keith chuckled softly.

He clambered off the bunk, shooting Keith a frosty look, and stripped down as fast as possible, awkwardly hopping on one floor and then the other to rid himself of his boots.

When he stood naked before the omega all hints of laughter were gone. Instead, Keith’s violet stare trailed along Shiro’s form like a physical caress, his glowing Glara gaze assessing the alpha and an appreciative hum fell from his lips.

“Well, that shut you up finally,” Shiro murmured, kissing along Keith’s left leg, nipping behind his knees, until he finally reached the omega’s gland on his inner thigh and suckled there.

Keith cried out, the sensation tugging and unexpected orgasm from him, the dark purple ridges running along his cock pulsing as cum dribbled over his engorged crown. Despite his ease finding release, Keith’s heat wouldn’t abate without an alpha.

Shiro’s mouth moved up his body, licking away the sticky mess as cum, oil, and sweat melded together, the aroma like a drug for the Altean as he turned himself over to the suffocating thirst that he intended to quench.

He made it to Keith’s mouth, smearing a kiss across the omega’s waiting lips. They arched toward one another, tongues colliding as Keith’s legs wrapped tight around Shiro’s waist, hips canting up so that the alpha’s thick length might slide between his cheeks.

“Fuck me already,” Keith growled, fangs bared as his eyes flashed gold, the weight of his heat resurfacing after his latest climax to spread a delicious ache through him.

Shiro bit down on the muscle connecting Keith’s neck and shoulder, the cry from the omega more satisfying than any before, his claws scratching up Shiro’s back. He aligned himself with Keith’s weeping entrance, slick covering his cock as he dipped his head past the tight ring of muscle.

He paused once inside, giving Keith a chance to adjust to his size and propping himself up on his hands to look down watch the omega’s expression as he entered him. Altean anatomy was much different than that of a Galra, longer, with a spade-shaped head that gave way to raised rings that matched the purple marking lining his shaft until they reached the hefty knot at his base.

“I’ve never mated with an Altean before,” Keith murmured, eyes glassy as he stared up at Shiro, his tail coming up around them to pass through Shiro’s white locks.

The long curls hung over Shiro’s face, tapering into a tight fade that was detailed with intricate, decorative lines as was the Altean style.

“I wonder if you’ll even be able to satisfy me.”

There was a note of challenge in Keith’s tone, his lips tugging into a sly grin, tail tickling Shiro’s pointed ear. It was enough to snap the last of Shiro’s consideration for Keith, the Galra pushing him past his normal bedroom etiquette and bringing out the savage alpha within that had been foaming at the mouth since he first stumbled across Keith’s scent.

Shiro drove in until his knot was pressed against Keith’s ass, victory roaring through his blood as the omega cried out his name, tightening around him.

“Shiro!”

“You like that?” he asked, his gravelly voice foreign to him.

Never had an omega driven him wild like this and he both admired and hated Keith for it, letting his emotions bubble over and strengthen the swing of his hips as he pummeled Keith’s ass.

“Don’t worry, I’ve got more for you,” Shiro groaned, closing his eyes and using his Altean abilities to grow his length further, expanding on his already-ample size to stretch Keith, the omega grinding against him.

Keith’s hands pawed at the Altean’s shoulders, his hips split wide as Shiro drove into him. His head thrashed back and forth, hair a tangled mass of dark curls fanned out beneath him. He curled his arms around Shiro’s neck, dragging him in close so his mouth could find the alpha’s throat gland.

He bit down, not enough to mark him or draw blood, but enough to release a flood of oil across his lips. He lapped it up, hips bucking against Shiro, drawing his length in deeper. Keith turned his head, letting his neck rub against the alpha’s, their glands connecting in an exquisite slide as their scents mingled together.

Shiro groaned doubling his efforts, his arms hooking beneath Keith’s knees to bend him in half as he bottomed out, pushing in his knot and pulling it back out, reveling in the way Keith tightened at his violent pace.    

The Altean’s sharp teeth imprinted on Keith’s lilac chest, bruises blooming along the path he made. It was heaven, Keith’s body climbing higher as he reached for an orgasm completely unlike anything he’d been able to give himself. His body was clenching, muscles straining as his a burning grew deep in the pit of his stomach, pulling from the tension in his thighs, his core.

In a swift movement that tipped Keith’s world, Shiro’s hands cradled Keith’s ass and lifted, the omega’s legs still folded between them, his ankles hooking over Shiro’s broad shoulders. Keith shrieked at the abrupt shift, then sighed as Shiro dropped him onto his length, arms guiding the omega as he pumped up, the display of strength something the Keith would marvel at later.

The new position hit all the right spots inside Keith and he felt the tension within him boil over, his entire body constricting, his toes curled as his orgasm rocked his slender frame. He tightened, convulsing around Shiro and he felt the alpha pump furiously, then groan as he buried himself deep within Keith, coming viciously.

For a moment, time stopped, and they shuddered against each other, Shiro’s arms straining as he held Keith up, the Galra coming apart so beautifully in his arms, his knot swelling to seal them together.

Shiro carefully untangled Keith’s legs, not wanting to pull out before his knot released and hurt the omega. He laid down, pulling Keith half on top of him, arms circled loosely around Keith’s waist.

Keith actually purred, his face nuzzling the crook of Shiro’s neck, fluffy ears tickling Shiro’s chin. The alpha within Shiro beat its chest with pride and he ignored how Keith’s body felt right within his embrace.

Soon they would have to break apart. Keith’s heat would pass and the rescue crew would arrive and they would head back to Altea, to their separate lives until an assignment brought them back together. He dismissed the pang of sadness that passed over him at the thought.

He hated Keith. They could never be a mated pair. Could never get along.

Even as the thoughts entered his mind, Shiro’s inner alpha protested, hackles raised, urging him to stake his claim. That this omega was _mine mine mine_.

“I’ve never felt anything like that before,” Keith whispered, and for a moment Shiro thought he was dreaming. “Wasn’t expecting it from you no less.”

A soft smile pulled at Shiro’s lips, the feisty omega crawling beneath his skin, Keith’s sleepy voice adorable, even as his barb ruffled Shiro’s feathers. But that was part of Keith’s appeal.

“For such a small omega, I’m surprised at how well you took me. If you ever need help in an emergency again, be sure to let me know.”

The blush that crossed Keith’s face was priceless and the alpha knew he was a goner. Regardless of what the future had in store for them, Shiro was certain that the battle ahead would be fun.

**Author's Note:**

> *dabs brow*
> 
> *soft whispers* 
> 
> yas
> 
> Let me know what you think!!
> 
> UPDATE! Continuation to this fic here: [Kinktober Day 25](https://archiveofourown.org/works/16421807)
> 
> As always, special thank you, also to my fabulous beta reader, [@thelastthingweneedisanentmoot](https://thelastthingweneedisanentmoot.tumblr.com/), who came up with the beautiful design for Shiro's hair THANK YOU QUEEN. 
> 
> If you wanna know more about my writing and Kinktober and participate in polls and raffles, follow me on Tumblr! -----> [@nihilistshiro](https://nihilistshiro.tumblr.com/)


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